We synthesized powder samples of Na$_{ x}$CoO$_{ 2}\cdot y$H$_{ 2}$O changing the volume of the water in the hydration process, then investigated their superconducting properties,. It was proved that the volume of water is one of key parameters to obtain a single phase of Na$_{ x}$CoO$_{ 2}\cdot y$H$_{ 2}$O with good superconducting properties. The transition temperature, $T_{ c}$, of the sample changed gradually while it was stored in the atmosphere of 70% humidity. Superconducting behavior under high magnetic field was very sensitive to $T_{ c}$. $H_{ c2}$ of a high quality sample with high $T_{ c}$ seemed very high.
Categories: cond-mat.supr-con